At least 120 people have been convicted for failing to complete their 2011 census forms, according to figures from the Office for National Statistics.

A number see themselves as conscientious objectors who are being prosecuted for refusing to fill out the forms because of the involvement of the defence contractor Lockheed Martin UK, which won the £150m contract to run the census.

By this week the ONS had referred 369 cases to the Crown Prosecution Service; of those, 157 prosecutions have been brought to court, with 120 resulting in convictions. Those found guilty face a maximum fine of £1,000 and a criminal record. The remaining 37 cases are classified as "conversions", where the defendant completed the census form at court.

The number of forms deliberately not filled in is unknown, but the ONS expected 24.5 million forms to be returned and received 22.9 million, a shortfall of 1.6 million. Once anomalies, such as vacant properties, have been taken into account, that number will fall.

John Marjoram, a local councillor and Green party mayor of the Cotswolds town of Stroud, refused to complete the census because of the involvement of Lockheed Martin UK. "When I was 18 I refused to fire a rifle on military service," he told the Guardian. "I couldn't live with myself if I collaborated with a military company."

Marjoram received his summons in early January. "Just after Christmas two people knocked on the door. I was busy cooking lunch. They were very nice and pleasant and understanding of where I was coming from, and the next thing was I got a summons."

Some of those being prosecuted have complained that they have been given court dates far from their homes. Marjoram's trial is set not for the magistrates court in Stroud, but 32 miles away in Bristol. John Voysey, who like Marjoram is a Quaker, has been summoned to appear in court in Wrexham on 1 February, 65 miles from his home in Ludlow.

"I've been extradited to a foreign country," said Voysey, 82, who first registered as a conscientious objector in 1947. "As soon as we heard that Lockheed Martin were involved we said we're not going to fill it in."

Lockheed Martin UK is a wholly owned subsidiary of the US defence contractor Lockheed Martin, which makes Trident nuclear missiles, cluster bombs and F-16 fighter jets. The UK subsidiary also provides the Royal Mail with address recognition technology. It was awarded the census contract in 2008 by the Labour government; overall the census was budgeted to have a total cost of £482m.

Groups opposed to Lockheed Martin's involvement in the census like to point to a quote from Lorraine Martin, a company vice-president, who said: "We want to know what's going on any time, any place on the planet." However a company spokesman said: "No Lockheed Martin employee has access to the [census] data."

Voysey expressed surprise that prosecutions were being pursued. "Before the election the Conservatives said this was going to be the last census. How can you punish people for something that is not going to happen again?"

Many of those accused under section eight of the 1920 Census Act say that they are being targeted because of their political beliefs. They wonder how the decision to prosecute almost 400 people was taken when so many forms were not returned.

A spokesman for the ONS, which runs the census in England and Wales, said: "Where there is evidence that householders have wilfully refused to complete a census form, the evidence has been presented to the Crown Prosecution Service, which then decides if a prosecution will go ahead."

The rate of prosecutions represents a sharp increase on the last census, in 2001. Then there were only 43 prosecutions, resulting in 38 convictions. Previous censuses have seen varying levels of prosecution. The highest number of successful prosecutions was 692 in 1981. In 1991 there were 342, while 1951 saw just 57.

Anti-census campaign group Count Me Out said the number of prosecutions showed it had been a mistake to give the contract to Lockheed Martin UK, adding that this had made the census less accurate.

• This article was amended on 31 January 2012. The original said that among the 120 people so far convicted, some might simply have forgotten to fill in their census form. This has been deleted. The Crown Prosecution Service has asked us to make clear that conviction of a person who had forgotten to complete the form is unlikely, as at least three further chances are offered to those facing a possible charge – first during the investigation process by the ONS, again when a summons is issued, and finally at court before their case goes ahead.